How rotary lime kilns work
A rotary lime kiln calcines limestone of 10 to 60 mm into quicklime at about 1,100 to 1,300°C, typically producing 150 to 1,000 tpd. Modern kilns have a vertical shaft preheater at the feed end, which brings the exit gas down to about 250 to 350°C; older long kilns without a preheater discharge gas at 600 to 900°C. Fuel is coal, petcoke, natural gas or oil, fired from the discharge end. The burnt lime drops into a vertical contact cooler, where air blown up through the lime bed cools it and becomes hot secondary air for the kiln. Calcination releases large volumes of CO₂, so the gas flow per tonne of product is high. The exhaust passes the preheater, a cooling stage and a bag filter before the ID fan.
Where the FD fan sits
- Inlet filter
- Cooler FD fan
- Cooler air plenum
- Lime bed in cooler
- Kiln hood
- Kiln
The fan sits at the base of the cooler and delivers air into plenums below the lime bed. The air rises through the burnt lime, reaching 600 to 800°C, and flows into the kiln hood as secondary air. Pressure at the fan outlet is typically 400 to 800 mmWC, depending on bed depth and lime size. A separate primary air fan supplies the burner, usually with a smaller flow at higher pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the cooler | 600 to 800°C, downstream of the fan, as secondary air |
| Dust | Lime dust around the cooler discharge; inlet filter or clean air intake needed |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Flow | Most of the combustion air for the kiln |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for rotary lime kilns
| Blade form | Backward curved, with a curve steep enough to hold flow as bed resistance changes |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, one fan per cooler section or a common fan with dampers |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on cooler air flow, linked to kiln firing |
| Also specified | Inlet filter, flow measurement and non-return damper |
What this FD fan has to cope with
Uneven air through the cooler
Lime segregates in the cooler, with fines on one side and coarse lumps on the other. Air takes the easy path and part of the lime leaves hot. Sectioned plenums with separate dampers or fans spread the air evenly.
Balance with the ID fan
The cooler fan pushes air into the hood while the ID fan draws it out. If the two are not coordinated the hood swings between pressure and suction. Control on hood pressure with the ID fan, and on air flow with the cooler fan, keeps them in step.
Hot lime discharge
If the cooler fan is undersized or loses flow, lime discharges too hot for conveyors and storage. The fan duty includes margin for maximum output and for coarser feed with higher bed resistance.
Energy and control
The cooler air fan runs continuously with the kiln. Its air is recovered as hot secondary air, so the right quantity improves fuel efficiency; excess cooler air is either vented or cools the flame. Speed control linked to kiln output keeps the air matched to the lime flow and saves fan power at reduced output.
ID fan and FD fan in rotary lime kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | At the cooler base, below the lime bed |
| What it handles | Kiln gas with calcination CO₂ | Ambient cooling and combustion air |
| Temperature at the fan | 180 to 230°C | Ambient |
| Main design concern | Large gas volume and filter temperature | Pressure through the lime bed |
| Control | Kiln hood pressure | Cooler air flow |
See also the Induced Draft (ID) Fan for rotary lime kilns.
Questions buyers ask
Which fan is the FD fan on a rotary lime kiln?
The cooler air fan is the main forced draft fan, as it supplies most of the combustion air through the cooler. A smaller primary air fan supplies the burner, and on coal-fired kilns the coal mill system adds its own air.
Does the lime cooler fan handle hot air?
No. It blows ambient air into the cooler. The air is heated by the lime after it leaves the fan, on its way to the kiln hood.
What pressure does a lime cooler fan need?
Typically 400 to 800 mmWC, depending on bed depth, lime size and cooler design. Fine or segregated lime raises the resistance and needs margin in the fan pressure.
Information needed to quote an FD fan for rotary lime kilns
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Cooler. Cooler type, number of sections, bed depth and lime size range.
- Air requirement. Cooler air flow and the secondary air temperature expected.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.